Although cigarette smoking has been linked to the etiology of several cancers, the relationship between smoking and breast cancer remains unclear. A woman's first pregnancy represents a period of rapid breast cell growth and differentiation and thus, a period of vulnerability to the influences of smoking or other exposures. During pregnancy, tobacco mutagens and free radical formation caused by smoking may affect rapidly growing breast tissue or act synergistically with elevated estrogens to increase breast cancer risk. Because breast tissue is less differentiated at the onset of first pregnancy, it may be more susceptible to mutagenesis than in subsequent pregnancies. The proposed population-based case-control study will utilize linked vital statistics - cancer registry data to test the hypothesis that cigarette smoking during first pregnancy is related to the risk of breast cancer. The specific aims of this study are to: 1)measure the risk of breast cancer associated with smoking during a first pregnancy relative to not smoking during the first pregnancy, and 2)evaluate possible differences in the relation between smoking during first pregnancy and breast cancer by tumor estrogen receptor status. To the extent possible, the study will also evaluate a possible dose response relation between the average number of cigarettes smoked per day during first pregnancy and breast cancer risk, and measure possible differences in the relation between smoking during first pregnancy and breast cancer risk by subject characteristics such as parity at the time of diagnosis and pre-pregnancy weight. This study will be among the first to examine smoking during first pregnancy and breast cancer risk. The clarification of the role of smoking during first pregnancy in breast cancer development will aid in understanding the complex etiology of breast cancer, and may identify a specific preventive strategy to help reduce breast cancer incidence.